Re: [RESEND PATCH] mm: page_alloc: validate buddy before check the migratetype

[Zi Yan <ziy@nvidia.com>]

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On 15 Jun 2022, at 2:47, Xianting Tian wrote:

> 在 2022/6/14 上午8:14, Zi Yan 写道:
>> On 13 Jun 2022, at 19:47, Guo Ren wrote:
>>
>>> On Tue, Jun 14, 2022 at 3:49 AM Zi Yan <ziy@nvidia.com> wrote:
>>>> On 13 Jun 2022, at 12:32, Guo Ren wrote:
>>>>
>>>>> On Mon, Jun 13, 2022 at 11:23 PM Zi Yan <ziy@nvidia.com> wrote:
>>>>>> Hi Xianting,
>>>>>>
>>>>>> Thanks for your patch.
>>>>>>
>>>>>> On 13 Jun 2022, at 9:10, Xianting Tian wrote:
>>>>>>
>>>>>>> Commit 787af64d05cd ("mm: page_alloc: validate buddy before check its migratetype.")
>>>>>>> added buddy check code. But unfortunately, this fix isn't backported to
>>>>>>> linux-5.17.y and the former stable branches. The reason is it added wrong
>>>>>>> fixes message:
>>>>>>>       Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable
>>>>>>>                           pageblocks with others")
>>>>>> No, the Fixes tag is right. The commit above does need to validate buddy.
>>>>> I think Xianting is right. The “Fixes:" tag is not accurate and the
>>>>> page_is_buddy() is necessary here.
>>>>>
>>>>> This patch could be applied to the early version of the stable tree
>>>>> (eg: Linux-5.10.y, not the master tree)
>>>> This is quite misleading. Commit 787af64d05cd applies does not mean it is
>>>> intended to fix the preexisting bug. Also it does not apply cleanly
>>>> to commit d9dddbf55667, there is a clear indentation mismatch. At best,
>>>> you can say the way of 787af64d05cd fixing 1dd214b8f21c also fixes d9dddbf55667.
>>>> There is no way you can apply 787af64d05cd to earlier trees and call it a day.
>>>>
>>>> You can mention 787af64d05cd that it fixes a bug in 1dd214b8f21c and there is
>>>> a similar bug in d9dddbf55667 that can be fixed in a similar way too. Saying
>>>> the fixes message is wrong just misleads people, making them think there is
>>>> no bug in 1dd214b8f21c. We need to be clear about this.
>>> First, d9dddbf55667 is earlier than 1dd214b8f21c in Linus tree. The
>>> origin fixes could cover the Linux-5.0.y tree if they give the
>>> accurate commit number and that is the cause we want to point out.
>> Yes, I got that d9dddbf55667 is earlier and commit 787af64d05cd fixes
>> the issue introduced by d9dddbf55667. But my point is that 787af64d05cd
>> is not intended to fix d9dddbf55667 and saying it has a wrong fixes
>> message is misleading. This is the point I want to make.
>>
>>> Second, if the patch is for d9dddbf55667 then it could cover any tree
>>> in the stable repo. Actually, we only know Linux-5.10.y has the
>>> problem.
>> But it is not and does not apply to d9dddbf55667 cleanly.
>>
>>> Maybe, Gregkh could help to direct us on how to deal with the issue:
>>> (Fixup a bug which only belongs to the former stable branch.)
>>>
>> I think you just need to send this patch without saying “commit
>> 787af64d05cd fixes message is wrong” would be a good start. You also
>> need extra fix to mm/page_isolation.c for kernels between 5.15 and 5.17
>> (inclusive). So there will need to be two patches:
>>
>> 1) your patch to stable tree prior to 5.15 and
>>
>> 2) your patch with an additional mm/page_isolation.c fix to stable tree
>> between 5.15 and 5.17.
>>
>>>> Also, you will need to fix the mm/page_isolation.c code too to make this patch
>>>> complete, unless you can show that PFN=0x1000 is never going to be encountered
>>>> in the mm/page_isolation.c code I mentioned below.
>>> No, we needn't fix mm/page_isolation.c in linux-5.10.y, because it had
>>> pfn_valid_within(buddy_pfn) check after __find_buddy_pfn() to prevent
>>> buddy_pfn=0.
>>> The root cause comes from __find_buddy_pfn():
>>> return page_pfn ^ (1 << order);
>> Right. But pfn_valid_within() was removed since 5.15. So your fix is
>> required for kernels between 5.15 and 5.17 (inclusive).
>>
>>> When page_pfn is the same as the order size, it will return the
>>> previous buddy not the next. That is the only exception for this
>>> algorithm, right?
>>>
>>>
>>>
>>>
>>> In fact, the bug is a very long time to reproduce and is not easy to
>>> debug, so we want to contribute it to the community to prevent other
>>> guys from wasting time. Although there is no new patch at all.
>> Thanks for your reporting and sending out the patch. I really
>> appreciate it. We definitely need your inputs. Throughout the email
>> thread, I am trying to help you clarify the bug and how to fix it
>> properly:
>>
>> 1. The commit 787af64d05cd does not apply cleanly to commits
>> d9dddbf55667, meaning you cannot just cherry-pick that commit to
>> fix the issue. That is why we need your patch to fix the issue.
>> And saying it has a wrong fixes message in this patch’s git log is
>> misleading.
>>
>> 2. For kernels between 5.15 and 5.17 (inclusive), an additional fix
>> to mm/page_isolation.c is also needed, since pfn_valid_within() was
>> removed since 5.15 and the issue can appear during page isolation.
>>
>> 3. For kernels before 5.15, this patch will apply.
>
> Zi Yan, Guo Ren,
>
> I think we still need some imporvemnt for MASTER branch, as we discussed above, we will get an illegal buddy page if buddy_pfn is 0,
>
> within page_is_buddy(), it still use the illegal buddy page to do the check. I think in most of cases, page_is_buddy() can return false,  but it still may return true with very low probablity.

Can you elaborate more on this? What kind of page can lead to page_is_buddy()
returning true? You said it is buddy_pfn is 0, but if the page is reserved,
if (!page_is_guard(buddy) && !PageBuddy(buddy)) should return false.
Maybe show us the dump_page() that offending page.

Thanks.

>
> I think we need to add some code to verify buddy_pfn in the first place.
>
> Could you give some suggestions for this idea?
>
>>
>>>>>>> Actually, this issue is involved by commit:
>>>>>>>       commit d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks")
>>>>>>>
>>>>>>> For RISC-V arch, the first 2M is reserved for sbi, so the start PFN is 512,
>>>>>>> but it got buddy PFN 0 for PFN 0x2000:
>>>>>>>       0 = 0x2000 ^ (1 << 12)
>>>>>>> With the illegal buddy PFN 0, it got an illegal buddy page, which caused
>>>>>>> crash in __get_pfnblock_flags_mask().
>>>>>> It seems that the RISC-V arch reveals a similar bug from d9dddbf55667.
>>>>>> Basically, this bug will only happen when PFN=0x2000 is merging up and
>>>>>> there are some isolated pageblocks.
>>>>> Not PFN=0x2000, it's PFN=0x1000, I guess.
>>>>>
>>>>> RISC-V's first 2MB RAM could reserve for opensbi, so it would have
>>>>> riscv_pfn_base=512 and mem_map began with 512th PFN when
>>>>> CONFIG_FLATMEM=y.
>>>>> (Also, csky has the same issue: a non-zero pfn_base in some scenarios.)
>>>>>
>>>>> But __find_buddy_pfn algorithm thinks the start address is 0, it could
>>>>> get 0 pfn or less than the pfn_base value. We need another check to
>>>>> prevent that.
>>>>>
>>>>>> BTW, what does first reserved 2MB imply? All 4KB pages from first 2MB are
>>>>>> set to PageReserved?
>>>>>>
>>>>>>> With the patch, it can avoid the calling of get_pageblock_migratetype() if
>>>>>>> it isn't buddy page.
>>>>>> You might miss the __find_buddy_pfn() caller in unset_migratetype_isolate()
>>>>>> from mm/page_isolation.c, if you are talking about linux-5.17.y and former
>>>>>> version. There, page_is_buddy() is also not called and is_migrate_isolate_page()
>>>>>> is called, which calls get_pageblock_migratetype() too.
>>>>>>
>>>>>>> Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks")
>>>>>>> Cc: stable@vger.kernel.org
>>>>>>> Reported-by: zjb194813@alibaba-inc.com
>>>>>>> Reported-by: tianhu.hh@alibaba-inc.com
>>>>>>> Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
>>>>>>> ---
>>>>>>>   mm/page_alloc.c | 3 +++
>>>>>>>   1 file changed, 3 insertions(+)
>>>>>>>
>>>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>>>>> index b1caa1c6c887..5b423caa68fd 100644
>>>>>>> --- a/mm/page_alloc.c
>>>>>>> +++ b/mm/page_alloc.c
>>>>>>> @@ -1129,6 +1129,9 @@ static inline void __free_one_page(struct page *page,
>>>>>>>
>>>>>>>                        buddy_pfn = __find_buddy_pfn(pfn, order);
>>>>>>>                        buddy = page + (buddy_pfn - pfn);
>>>>>>> +
>>>>>>> +                     if (!page_is_buddy(page, buddy, order))
>>>>>>> +                             goto done_merging;
>>>>>>>                        buddy_mt = get_pageblock_migratetype(buddy);
>>>>>>>
>>>>>>>                        if (migratetype != buddy_mt
>>>>>>> --
>>>>>>> 2.17.1
>>>>>> --
>>>>>> Best Regards,
>>>>>> Yan, Zi
>>>>>
>>>>>
>>>>> --
>>>>> Best Regards
>>>>>   Guo Ren
>>>>>
>>>>> ML: https://lore.kernel.org/linux-csky/
>>>> --
>>>> Best Regards,
>>>> Yan, Zi
>>>
>>>
>>> -- 
>>> Best Regards
>>>   Guo Ren
>>>
>>> ML: https://lore.kernel.org/linux-csky/
>> --
>> Best Regards,
>> Yan, Zi

--
Best Regards,
Yan, Zi

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